An Asynchronous Transfer Mode (ATM) is a switching and multiplexing technique designed for transmitting digital information, such as data, video, and voice, at high speed, with low delay, over a telecommunications network. The telecommunications network, for example an ATM network, includes a number of switching nodes coupled through communication links. In the ATM network, bandwidth capacity is allocated to fixed-sized data units named “cells.” The communication links transport the cells from a switching node to another. These communication links can support many virtual connections, also named channels, between the switching nodes. The virtual connections assure the flow and delivery of information contained in the cells.
Each cell contains a cell header and cell data. The cell header includes information necessary to identify the destination of that cell. The components of the cell header include, among other things, a Virtual Channel Identifier (VCI) and a Virtual Path Identifier (VPI), for collectively identifying an ATM connection for that particular cell, and a Payload Type Identifier (PTI), for indicating whether the cell is sent from one user to another, whether cell data refers to administration or management traffic, and whether congestion is present within the network.
The ATM Forum, which is a user and vendor group establishing ATM standards, has also defined several ATM class of service categories, used in characterization of a virtual connection, for example, (1) a Constant Bit Rate (CBR), which supports a constant or guaranteed rate to transport services, such as video or voice, as well as circuit emulation, which requires rigorous timing control and performance parameters; (2) a Variable Bit Rate (VBR), real time and non real time, which supports variable bit rate data traffic with average and peak traffic parameters; (3) an Available Bit Rate (ABR), which supports feedback to control the source rate in response to changed characteristics in the network; and (4) an Unspecified Bit Rate (UBR).